Absorption heat pumps are gaining increased attention as an environmentally friendly replacement for the CFC-based vapor-compression systems that are used in residential and commercial air-conditioning. These heat pumps rely heavily on internal recuperation to yield high performance. Several studies have shown that the high coefficients of performance of these thermodynamic cycles cannot be realized without the development of practically feasible and compact heat exchangers. While significant research has been done on absorption cycle simulation, innovations in component development have been rather sparse, in spite of the considerable influence of component performance on system viability. There have been some advances in the design of compact geometries for components such as condensers and in the use of fluted tubes to enhance single-phase components such as solution-solution heat exchangers. But absorption and desorption processes involve simultaneous heat and mass transfer in binary fluids. For example, in a Lithium Bromide-Water (LiBr-H2O) cycle, absorption of water vapor in concentrated LiBr-H2O solutions occurs in the absorber with the associated rejection of heat to the ambient or an intermediate fluid. Successful designs for such binary fluid heat and mass exchangers must address the following often contradictory requirements:
low heat and mass transfer resistances for the absorption/desorption side
adequate transfer surface area on both sides.
low resistance of the coupling fluidxe2x80x94designs have been proposed in the past that enhance absorption/desorption processes, but fail to reduce the single-phase resistance on the other side, resulting in large components.
low coupling fluid pressure dropxe2x80x94to reduce parasitic power consumption
low absorption side pressure dropxe2x80x94this is essential because excessive pressure drops, encountered in forced-convective flow at high mass fluxes, decrease the saturation temperature and temperature differences between the working fluid and the heat sink.
Most of the available absorber/desorber concepts fall short in one or more of the above-mentioned criteria essential for good design.
It is therefore a principal object of this invention to provide a method and means for miniaturization of binary-fluid heat and mass exchangers which will permit designs that are compact, modular, versatile, easy to fabricate and assemble, and wherein use can be made of existing heat transfer technology without special surface preparation.
These and other objects will be apparent to those skilled in the art.
This invention addresses the deficiencies of currently available designs. It is an extremely simple geometry that is widely adaptable for a variety of miniaturized absorption system components. It can be used for fluid pairs with non-volatile and volatile absorbents. It promotes high heat and mass transfer rates through flow mechanisms such as counter-current vapor-liquid flow, vapor shear, droplet entrainment, adiabatic absorption between tubes, species concentration redistribution due to liquid droplet impingement, significant interaction between vapor and liquid flow around adjacent tubes in the transverse and vertical directions, and other deviations from idealized falling films. It ensures uniform distribution of the liquid and vapor films and high wettability of the transfer surfaces.
Short lengths of very small diameter tubes are placed in a square array, with several such arrays being stacked vertically. Successive tube arrays are oriented in a transverse orientation perpendicular to the tubes in adjacent levels. In an absorber application, the liquid solution flows in the falling-film mode counter-current to the coolant through the tube rows. Vapor flows upward through the lattice formed by the tube banks, counter-current to the falling solution. The effective vapor-solution contact minimizes heat and mass transfer resistances, the solution and vapor streams are self-distributing, and wetting problems are minimized. Coolant-side heat transfer coefficients are extremely high without any passive or active surface treatment or enhancement, due to the small tube diameter.